Is DesignBuilder suitable for modelling natural ventilation?
Posted by Andy Tindale on 12-Jan-13 11:54 AM

Yes, DesignBuilder is well suited to modelling natural ventilation at simple (scheduled) and detailed (calculated) levels:


You define the natural ventilation rate and its operation. The advantages of 'Scheduled' natural ventilation are:

  • Assuptions about directly entered natural ventilation airflow rates are easy to document for design calculations and
  • Simulations are much faster.


You define opening and crack sizes, pressure coefficients and control strategies and EnergyPlus/AIRNET calculates the natural ventilation flow rates including consideration of wind and buoyancy pressure effects, natural ventilation setpoint temperatures and flow between zones. DesignBuilder automatically sets up default Wind Pressure Coefficients using standard AIVC data. The AIVC WPC are good for early design stage natural ventilation studies but should be replaced by WPC from CFD calculations or wind tunnel measurements for more detailed studies, high rise buildings or buildings using other features such as wind pressure induction vents which require special treatment. DB Version 2 will have integrated CFD for calculating WPC and also will take E+ output as boundary conditions for internal airflow analysis.

Advantages of 'Calculated' natural ventilation are:

  • Results are potentially more accurate because they take into account wind and buoyancy pressure effects using physically meaningful equations,
  • No need for 'difficult to justify' assumptions about scheduled natural ventilation airflow rates.

One approach combines both methods to create a realistic model that simulates fast:

  1. Set up a 'Calculated' natural ventilation model and use it to estimate typical natural ventilation air change rates for design and typical conditions in each zone. Use hourly calculated ACH results to do this.
  2. Switch natural ventilation model option to 'Scheduled'.
  3. Feed typical calculated ACH data into natural ventilation airflow rate data on HVAC tab. 

You can select 'Scheduled' or 'Calculated' natural ventilation from the Model Options dialog.

Check the Program Help for more information on natural ventilation modelling.

See also: Using the 'Calculated' Natural ventilation Model option